Semiconductor device and method of manufacturing semiconductor device

ABSTRACT

A semiconductor device, including a first semiconductor layer of the first conductivity type formed on a semiconductor substrate, a first semiconductor region of the first conductivity type, a first base region of a second conductivity type and a first base region of a second conductivity type that are respectively selectively provided in the first semiconductor layer, a second semiconductor layer of the second conductivity type provided on the first semiconductor layer, a second semiconductor region of the first conductivity type selectively provided in the second semiconductor layer, a trench that penetrates the second semiconductor layer and the second semiconductor region, a gate electrode provided in the trench via a gate insulating film, an interlayer insulating film provided on the gate electrode, a first electrode in contact with the second semiconductor layer and the second semiconductor region, and a second electrode provided on a back surface of the semiconductor substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2019-025998, filed on Feb. 15,2019, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Embodiments of the invention relate to a semiconductor device and amethod of manufacturing a semiconductor device.

2. Description of the Related Art

Conventionally, to reduce element ON resistance in a power semiconductorelement, a vertical metal oxide semiconductor field effect transistor(MOSFET) having a trench structure is fabricated (manufactured). In avertical MOSFET, cell density per unit area may be increased more for atrench structure in which a channel is formed orthogonally to asubstrate surface than for a planar structure in which the channel isformed parallel to the substrate surface. Accordingly, a trenchstructure enables current density per unit area to be increased morethan does a planar structure and is more advantageous in terms of cost.

Nonetheless, a vertical MOSFET having a trench structure has a structurein which an entire inner wall region of a trench is covered by a gateinsulating film to form the channel in an orthogonal direction and sincea bottom portion of the gate insulating film at a bottom of the trenchis near a drain electrode, high electric field is easily applied to aportion of the gate insulating film at the bottom of the trench. Inparticular, since an ultra-high voltage element is fabricated using awide bandgap semiconductor material (semiconductor material having abandgap wider than that of silicon, for example, silicon carbide (SiC)),the gate insulating film at the bottom of the trench is adverselyaffected, thereby greatly reducing reliability.

As a method to solve such problems and to mitigate electric fieldstrength at the bottom of the trench, a structure has been proposed inwhich a p-type region is formed that is in contact with a p-type baseregion and reaches a position deeper than is a bottom of a trench, a pnjunction is formed at a position deeper than the bottom of the trenchand near the trench, and a p-type region is further formed at the bottomof the trench.

FIG. 17 is a cross-sectional view of a structure of a conventionalsilicon carbide semiconductor device. A trench-type MOSFET 150 isdepicted as a conventional silicon carbide semiconductor device. In thetrench-type MOSFET 150, at a front surface of an n⁺-type silicon carbidesubstrate 101, an n⁻-type silicon carbide epitaxial layer 102 isdeposited. In an upper portion of the n⁻-type silicon carbide epitaxiallayer 102, opposite a lower portion thereof facing the n⁺-type siliconcarbide substrate 101, an n-type high-concentration region 105 isprovided. Further, in the n-type high-concentration region 105, a firstp⁺-type base region 104 is selectively provided so as to underlie abottom of a trench 116 overall. In an upper portion of the n-typehigh-concentration region 105, opposite a lower portion thereof facingtoward the n⁺-type silicon carbide substrate 101, a second p⁺-type baseregion 103 is selectively provided.

In the conventional trench-type MOSFET 150, a p-type base layer 106, ann⁺-type source region 107, a p⁺⁺-type contact region 108, a gateinsulating film 109, a gate electrode 110, an interlayer insulating film111, a source electrode 112, a rear electrode 113, a source electrodepad 114, and a drain electrode pad 115 are further provided.

In the trench-type MOSFET 150 of the configuration in FIG. 17, pnjunctions between p-type regions (the first p⁺-type base region 104 andthe second p⁺-type base region 103) and the n-type high-concentrationregion 105 are positioned deeper than the trench 116 and therefore,electric field concentrates at borders between the p-type regions andthe n-type high-concentration region 105, thereby enabling electricfield concentration at the bottom of the trench 116 to be mitigated.

Further, in the conventional structure depicted in FIG. 17, the firstp⁺-type base region 104 is formed so as to be positioned beneath thetrench 116 and therefore, the first p⁺-type base region 104 has to beformed widely with consideration of process margin. Thus, when cellpitch is to be reduced, photolithography of higher precision isnecessary, increasing the degree of manufacturing difficulty andlimiting the extent of reductions in element size. On the other hand,when the first p⁺-type base region 104 is not provided, the electricfield concentration at the bottom of the trench 116 cannot be mitigatedand thus, targeted breakdown voltage cannot be realized.

Therefore, a technique has been proposed in which in a vertical MOSFETwith a trench structure having a planar pattern of a striped shape, ap⁺-type base region having a striped shape orthogonal to a directionalong which the striped shape of the trench extends is provided beneaththe trench.

FIG. 18 is a perspective view of another structure of the conventionalsilicon carbide semiconductor device. FIG. 19 is a cross-sectional viewalong cutting line A-A′ in FIG. 18 depicting the other structure of theconventional silicon carbide semiconductor device. FIG. 20 is a plansectional view of a portion along cutting line B-B′ in FIG. 18 depictingthe other structure of the conventional silicon carbide semiconductordevice. As depicted in FIG. 18 to FIG. 20, in the n-typehigh-concentration region 105, the first p⁺-type base region 104 isselectively provided orthogonally to the trench 116 so as to partiallyunderlie the bottom of the trench 116.

In the trench-type MOSFET 150 of the configuration in FIGS. 18 to 20 aswell, the pn junction between the first p⁺-type base region 104 and then-type high-concentration region 105 is positioned deeper than thetrench 116 and therefore, electric field concentrates at the borderbetween the first p⁺-type base region 104 and the n-typehigh-concentration region 105, thereby enabling electric fieldconcentration at the bottom of the trench 116 to be mitigated.

Further, a technique has been proposed in which in a first siliconcarbide layer, a first impurity layer in which a bottom of a trench isformed and a second impurity layer are formed, a first-conductivity-typeimpurity concentration directly beneath the first impurity layer in thefirst silicon carbide layer is set lower than a first-conductivity-typeimpurity concentration directly beneath the second impurity layer in thefirst silicon carbide layer, whereby breakdown voltage of the firstimpurity layer is made higher than breakdown voltage of the secondimpurity layer and avalanche breakdown is selectively caused in thesecond impurity layer (for example, refer to Japanese Laid-Open PatentPublication No. 2018-19046).

Further, a technique has been proposed in which a base region of asecond conductivity type is selectively provided in a surface layer of awide bandgap semiconductor layer of a first conductivity type and atrench having a planar pattern of a striped shape is provided while abase region is periodically provided in a direction parallel to thetrench and at a bottom of the trench, a portion of the base regionextends in a direction parallel to the trench, thereby connecting thebase regions to each other (for example, refer to Japanese Laid-OpenPatent Publication No. 2018-82055).

SUMMARY OF THE INVENTION

According to an embodiment of the invention, a semiconductor deviceincludes a semiconductor substrate of a first conductivity type, havinga front surface and a back surface opposite to each other; a firstsemiconductor layer of the first conductivity type, provided on thefront surface of the semiconductor substrate and having a first sidefacing the semiconductor substrate and a second side opposite to thefirst side, an impurity concentration of the first semiconductor layerbeing lower than that of the semiconductor substrate; a firstsemiconductor region of the first conductivity type, selectivelyprovided in the first semiconductor layer, a peak impurity concentrationof the first semiconductor region being higher than that of the firstsemiconductor layer; a first base region of a second conductivity type,selectively provided in the first semiconductor layer; a secondsemiconductor layer of the second conductivity type, provided on asurface of the first semiconductor layer at the second side thereof; asecond semiconductor region of the first conductivity type, selectivelyprovided in the second semiconductor layer at a surface thereof; atrench that penetrates the second semiconductor layer and the secondsemiconductor region, and reaches the first semiconductor layer; a gateelectrode provided in the trench, via a gate insulating film; aninterlayer insulating film provided on the gate electrode;

a first electrode in contact with the second semiconductor layer and thesecond semiconductor region; and a second electrode provided on the backsurface of the semiconductor substrate. The trench includes a pluralityof trench portions, each of which extends in a first direction. Thefirst semiconductor region is provided between adjacent two of theplurality of trench portions, and is closer to the second electrode thanis the first base region. The first base region extends along a seconddirection that intersects the first direction, at a depth that is thesame as a bottom of the trench, or is closer to the second electrodethan is the bottom of the trench.

In the embodiment, the first base region includes a plurality of firstbase region portions, and an interval between any adjacent two of thefirst base region portions is shorter than an interval between anyadjacent two of the trench portions.

In the embodiment, the first base region includes a plurality of firstbase region portions, and an interval between any adjacent two of thefirst base region portions is in a range from 0.8 μm to 5 μm.

In the embodiment, a second base region in contact with the bottom ofthe trench, in a direction parallel to the first direction.

In the embodiment, the second base region has a same impurityconcentration as that of the first base region.

In the embodiment, the first base region includes a plurality of firstbase region portions. The second base region includes a plurality ofsecond base region portions, and an interval between any adjacent two ofthe second base region portions is of a same width as an intervalbetween any adjacent two of the first base region portions.

In the embodiment, the first base region includes a plurality of firstbase region portions. The second base region includes a plurality ofsecond base region portions. The first semiconductor region includes aplurality of first semiconductor region portions, and a width of each ofthe first base region portions or a width of each of the second baseregion portions is wider than a width of each of the first semiconductorregion portions.

In the embodiment, the semiconductor device further includes a thirdsemiconductor region of the second conductivity type, selectivelyprovided in the second semiconductor layer. The first semiconductorregion is provided at a position facing the third semiconductor regionin a depth direction.

In the embodiment, the first semiconductor region includes a pluralityof first semiconductor region portions, each between adjacent two of theplurality of trench portions, and extending in a direction parallel tothe first direction.

In the embodiment, the first semiconductor region is provided separatedfrom the first base region.

In the embodiment, an impurity concentration of the first semiconductorregion is lower than an impurity concentration of the first base region.

In the embodiment, an impurity concentration of the first semiconductorregion is in a range from 1×10¹⁷/cm³ to 1×10²⁰/cm³.

In the embodiment, an angle between the first and second directions isone of 30 degrees, 60 degrees, 90 degrees, 120 degrees or 150 degrees.

In the embodiment, the angle has a margin of ±4 degrees.

According to another embodiment, a method of manufacturing asemiconductor device includes providing a semiconductor substrate of afirst conductivity type, the semiconductor substrate having a frontsurface and a back surface opposite to each other; forming, on the frontsurface of the semiconductor substrate, a first semiconductor layer ofthe first conductivity type, the first semiconductor layer having animpurity concentration lower than that of the semiconductor substrateand having a first side facing the semiconductor substrate and a secondside opposite to the first side; selectively forming, in the firstsemiconductor layer, a first semiconductor region of the firstconductivity type, the first semiconductor region having a peak impurityconcentration higher than that of the first semiconductor layer;selectively forming, in the first semiconductor layer, a first baseregion of a second conductivity type; forming, on a surface of the firstsemiconductor layer at the second side thereof, a second semiconductorlayer of the second conductivity type; selectively forming, in thesecond semiconductor layer at a surface thereof, a second semiconductorregion of the first conductivity type; forming a trench that penetratesthe second semiconductor layer and the second semiconductor region, andreaches the first semiconductor layer; forming, in the trench, a gateelectrode via a gate insulating film; forming an interlayer insulatingfilm on the gate electrode; forming a first electrode in contact withthe second semiconductor layer and the second semiconductor region; andforming a second electrode on the back surface of the semiconductorsubstrate. Forming a trench includes forming a plurality of trenchportions, each of which extends in a first direction. Forming the firstsemiconductor region includes forming a plurality of first semiconductorregion portions, each between adjacent two of the trench portions, thefirst semiconductor region being closer to the second electrode than isthe first base region. Forming the first base region includes formingthe first base region along a second direction intersecting the firstdirection, at a depth that is the same as a bottom of the trench, or iscloser to the second electrode than is the bottom of the trench.

Objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a structure of a silicon carbidesemiconductor device according to a first embodiment, along cutting lineA-A′ in FIG. 3A.

FIG. 2 is a cross-sectional view of the structure of the silicon carbidesemiconductor device according to the first embodiment, along cuttingline B-B′ in FIG. 3A.

FIG. 3A is a plan sectional view of the structure of the silicon carbidesemiconductor device according to the first embodiment.

FIG. 3B is a plan sectional view of a different structure in FIG. 3A.

FIG. 4 is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 5A is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 5B is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 6A is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 6B is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 7A is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 7B is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 8 is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 9 is a cross-sectional view of the silicon carbide semiconductordevice according to the first embodiment during manufacture.

FIG. 10 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a second embodiment, along cuttingline A-A′ in FIG. 12.

FIG. 11 is a cross-sectional view of the structure of the siliconcarbide semiconductor device according to the second embodiment, alongcutting line B-B′ in FIG. 12.

FIG. 12 is a plan sectional view of the structure of the silicon carbidesemiconductor device according to the second embodiment.

FIG. 13 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a third embodiment.

FIG. 14 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a fourth embodiment, along cuttingline B-B′ in FIG. 15.

FIG. 15 is a plan sectional view of the structure of the silicon carbidesemiconductor device according to the fourth embodiment.

FIG. 16 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a fifth embodiment.

FIG. 17 is a cross-sectional view of a structure of a conventionalsilicon carbide semiconductor device.

FIG. 18 is a perspective view of another structure of the conventionalsilicon carbide semiconductor device.

FIG. 19 is a cross-sectional view along cutting line A-A′ in FIG. 18depicting the other structure of the conventional silicon carbidesemiconductor device.

FIG. 20 is a plan sectional view of a portion along cutting line B-B′ inFIG. 18 depicting the other structure of the conventional siliconcarbide semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

First, problems associated with the conventional techniques will bedescribed. In the conventional silicon carbide semiconductor devicedepicted in FIG. 17, breakdown voltage with respect to drain voltage atthe first p⁺-type base region 104 of the bottom of the trench 116 andbreakdown voltage with respect to drain voltage at the second p⁺-typebase region 103 beneath the p⁺⁺-type contact region 108 aresubstantially the same. Therefore, the breakdown voltage of the firstp⁺-type base region 104 at the bottom of the trench 116 may be less thanthe breakdown voltage of the second p⁺-type base region 103 beneath thep⁺⁺-type contact region 108; and in this case, breakdown voltage of theelement overall is determined by the breakdown voltage of the bottom ofthe trench 116.

Accordingly, avalanche breakdown occurs at the bottom of the trench 116,whereby carriers are injected to the gate insulating film 109 at thebottom of the trench 116 and the injected carriers are captured in acarrier trap in the gate insulating film 109, thereby modulating localelectric field in the gate insulating film 109. Further, due tovariation of the local electric field, the local electric field becomesstronger, whereby a vicious circle arises in which further carrierinjection occurs and the gate insulating film 109 may be destroyed.

Embodiments of a semiconductor device and a method of manufacturing asemiconductor device according to the present invention will bedescribed in detail with reference to the accompanying drawings. In thepresent description and accompanying drawings, layers and regionsprefixed with n or p mean that majority carriers are electrons or holes.Additionally, + or − appended to n or p means that the impurityconcentration is higher or lower, respectively, than layers and regionswithout + or −. Cases where symbols such as n's and p's that include +or − are the same indicate that concentrations are close and therefore,the concentrations are not necessarily equal. In the description of theembodiments below and the accompanying drawings, main portions that areidentical will be given the same reference numerals and will not berepeatedly described. Further, in the present description, when Millerindices are described, “−” means a bar added to an index immediatelyafter the “−”, and a negative index is expressed by prefixing “−” to theindex. Further, with consideration of variation in manufacturing,description indicating the same or equal may be within 5%.

A semiconductor device according to the present invention is configuredusing a wide bandgap semiconductor. In a first embodiment, a siliconcarbide semiconductor device fabricated using, for example, siliconcarbide (SiC) as the wide bandgap semiconductor will be described takinga MOSFET as an example. FIG. 1 is a cross-sectional view of a structureof the silicon carbide semiconductor device according to the firstembodiment, along cutting line A-A′ in FIG. 3A. FIG. 2 is across-sectional view of the structure of the silicon carbidesemiconductor device according to the first embodiment, along cuttingline B-B′ in FIG. 3A. FIGS. 1 to 3B depict an example of a trench-typeMOSFET 50.

As depicted in FIGS. 1 and 2, in the silicon carbide semiconductordevice according to the first embodiment, at a first main surface (frontsurface), for example, a (0001) plane (Si-face), of an n⁺-type siliconcarbide substrate (semiconductor substrate of a first conductivity type)1, an n⁻-type silicon carbide epitaxial layer (first semiconductor layerof the first conductivity type) 2 is deposited.

The n⁺-type silicon carbide substrate 1 is a silicon carbide singlecrystal substrate. The n⁻-type silicon carbide epitaxial layer 2 has animpurity concentration lower than that of the n⁺-type silicon carbidesubstrate 1 and, for example, is a low-concentration n-type drift layer.At a surface of the n⁻-type silicon carbide epitaxial layer 2, on afirst side thereof opposite a second side thereof facing the n⁺-typesilicon carbide substrate 1, an n-type high-concentration region (firstsemiconductor layer of the first conductivity type) 5 may be provided.The n-type high-concentration region 5 is a high-concentration n-typedrift layer having an impurity concentration lower than that of then⁺-type silicon carbide substrate 1 and higher than that of the n⁻-typesilicon carbide epitaxial layer 2.

In an upper portion of the n⁻-type silicon carbide epitaxial layer 2,opposite a lower portion thereof facing the n⁺-type silicon carbidesubstrate 1, a p-type base layer (second semiconductor layer of a secondconductivity type) 6 is provided. Hereinafter, the n⁺-type siliconcarbide substrate 1, the n⁻-type silicon carbide epitaxial layer 2, andthe p-type base layer 6 collectively are a silicon carbide semiconductorbase.

At a second main surface (back surface, i.e., back surface of thesilicon carbide semiconductor base) of the n⁺-type silicon carbidesubstrate 1, a drain electrode forming a rear electrode (secondelectrode) 13 is provided. At a surface of the rear electrode 13, adrain electrode pad 15 is provided.

At a first main surface side (side with the p-type base layer 6) of thesilicon carbide semiconductor base, a trench structure is formed. Inparticular, from a surface of the p-type base layer 6 on a first side(first main surface side of the silicon carbide semiconductor base)thereof opposite a second side thereof facing toward the n⁺-type siliconcarbide substrate 1, a trench 16 penetrates the p-type base layer 6 andreaches the n-type high-concentration region 5 (when the n-typehigh-concentration region 5 is not provided, the n⁻-type silicon carbideepitaxial layer 2, hereinafter, (2)). The trench 16 has a planar patternof a striped shape. Along an inner wall of the trench 16, a gateinsulating film 9 is formed at side walls and a bottom of the trench 16,and a gate electrode 10 is formed on the gate insulating film 9 in thetrench 16. The gate electrode 10 is insulated from the n⁻-type siliconcarbide epitaxial layer 2 and the p-type base layer 6 by the gateinsulating film 9. A portion of the gate electrode 10 may protrude froma top (side where a source electrode pad 14 is provided) of the trench16, toward the source electrode pad 14.

In the n⁻-type silicon carbide epitaxial layer 2, a first p⁺-type baseregion 4 that is in contact with the bottom of the trench 16 is providedalong a direction orthogonal to a direction in which the striped shapeof the trench 16 extends. In FIG. 1, while the first p⁺-type base region4 is in contact with the bottom of the trench 16, the first p⁺-type baseregion 4 may be provided at a position deeper than the bottom of thetrench 16. The first p⁺-type base region 4 reaches a position deepertoward a drain side than is the bottom of the trench 16. Provision ofthe first p⁺-type base region 4 enables formation of a pn junctionbetween the first p⁺-type base region 4 and the n-typehigh-concentration region 5 (2) at a position near the bottom of thetrench 16, in a depth direction (direction from the source electrode pad14 toward the drain electrode pad 15). In this manner, the pn junctionbetween the first p⁺-type base region 4 and the n-typehigh-concentration region 5 (2) is formed, whereby high electric fieldmay be prevented from being applied to the gate insulating film 9 at thebottom of the trench 16. Therefore, even when a wide bandgapsemiconductor is used as a semiconductor material, high breakdownvoltage is possible. In the first embodiment, a case is depicted inwhich intersection of the direction along which the striped shape of thetrench 16 extends and the direction along which the first p⁺-type baseregion 4 is orthogonal, i.e., 90 degrees. Other than orthogonal (90degrees), an angle of the intersection may be any one of 30 degrees, 60degrees, 120 degrees, or 150 degrees. By setting the angle ofintersection to any one of 30 degrees, 60 degrees, 90 degrees, 120degrees, and 150 degrees, the hexagonal crystalline silicon carbide andsurface orientation are aligned, mobility is easy, and a tradeoffbetween Vth threshold and ON resistance RonA may be optimized. Further,with consideration of manufacturing variation, when the angle ofintersection is 30 degrees, 60 degrees, 90 degrees, 120 degrees, or 150degrees, a margin thereof is within ±4 degrees and may be within ±2thereof.

In the first embodiment, an n⁺-type region (first semiconductor regionof the first conductivity type) 17 having a peak impurity concentrationhigher than the n-type high-concentration region 5 (2) is provided inthe n-type high-concentration region 5 (2), between the trench 16 and anadjacent trench 16, at a position deeper than the first p⁺-type baseregion 4. The peak impurity concentration of the n⁺-type region 17suffices to be higher than the impurity concentration of the n-typehigh-concentration region 5 (2) and the impurity concentration of allregions of the n⁺-type region 17 needs not be higher than of the n-typehigh-concentration region 5 (2). Further, when the n-typehigh-concentration region 5 is not provided, the n⁺-type region 17 has apeak impurity concentration that is higher than the impurityconcentration of the n⁻-type silicon carbide epitaxial layer 2.Moreover, the n⁺-type region 17 may have an impurity concentration thatis lower than the impurity concentration of the first p⁺-type baseregion 4.

The n⁺-type region 17 enables setting the part that determines elementbreakdown voltage to be the first p⁺-type base region 4 between thetrench 16 and the adjacent trench 16, and not the first p⁺-type baseregion 4 at the bottom of the trenches 16. In other words, the breakdownvoltage of the first p⁺-type base region 4 at the bottom of the trench16 may be higher than the breakdown voltage of the first p⁺-type baseregion 4 between the trench 16 and the adjacent trench 16.

Therefore, by selectively concentrating electric field at the n⁺-typeregion 17, avalanche breakdown is caused and the generation of avalanchecurrent at the bottom of the trench 16 may be suppressed. Therefore, forexample, modulation of local electric field in the gate insulating film9 due to carrier injection to the gate insulating film 9 at the bottomof the trench 16, and insulation breakdown of the gate insulating film 9may be suppressed. In this manner, the n⁺-type region 17 enablesavalanche beneath the trench to be avoided and reliability to beincreased.

Further, the impurity concentration of the n⁺-type region 17 may be in arange from 1×10¹⁷/cm³ to 1×10²⁰/cm³. When the impurity concentration isless than 1×10¹⁷/cm³, electric field cannot be concentrated at then⁺-type region 17 and when higher than 1×10²⁰/cm³, element performancefalls below a rated value.

FIG. 3A is a plan sectional view of the structure of the silicon carbidesemiconductor device according to the first embodiment. FIG. 3Aschematically depicts a positional relationship between the trench 16,the first p⁺-type base region 4, and the n⁺-type region 17. As depictedin FIG. 3A, the trench 16 and the first p⁺-type base region 4 both havestriped shapes and directions in which the striped shapes extend areorthogonal to each other. The n⁺-type region 17 is selectively providedin the first p⁺-type base region 4 between the trench 16 and the trench16. In this manner, in the first embodiment, the trench 16 and the firstp⁺-type base region 4 are orthogonal to each other and therefore, evenwhen cell pitch is reduced, the trench 16 and the first p⁺-type baseregion 4 do not become misaligned, whereby a state in which the firstp⁺-type base region 4 is not positioned at the bottom of the trench 16may be prevented. With consideration of manufacturing variation,orthogonality is within ±4 degrees and may be within ±2 degrees. FIG. 3Bis a plan sectional view of a different structure in FIG. 3A. While FIG.3A depicts a case in which the intersection between the direction inwhich the striped shape of the trench 16 extends and the direction inwhich the first p⁺-type base region 4 extends is orthogonal, i.e., 90degrees, FIG. 3B depicts a case in which the intersecting angle is 30degrees. Further, an interval w1 between the trenches 16 may be widerthan an interval w2 between the first p⁺-type base region 4 and anadjacent first p⁺-type base region 4 (w1>w2). The interval w1 betweenthe trenches 16 is a distance between a center of the trench 16 and acenter of the adjacent trench 16 that is adjacent to the trench 16.Further, the interval w2 between the first p⁺-type base regions 4 is adistance between a center of the first p⁺-type base region 4 and acenter of the adjacent first p⁺-type base region 4 that is adjacent tothe first p⁺-type base region 4. The n⁺-type region 17 is provided neara center between the trench 16 and the adjacent trench 16. Therefore,the interval w1 between the trenches 16 is about an interval w3 betweenn⁺-type regions 17. Further, the interval w3 between the n⁺-type regions17 is a distance between a center of the n⁺-type region 17 and thecenter of an adjacent n⁺-type region 17 that is adjacent to the n⁺-typeregion 17. In this manner, as compared to the conventional structure inwhich the interval of the trenches 116 and the interval of the firstp⁺-type base regions 104 are equal, the ON resistance and the electricfield may be concurrently reduced and breakdown voltage of the firstp⁺-type base region 4 between the trenches 16 may be easily reduced.

Further, the interval w2 of the first p⁺-type base regions 4 may be in arange from 0.8 μm to 5 μm because when less than 0.8 μm, resistance of ajunction field effect transistor (JFET) region increases and whengreater than 5 μm, an effect of protecting the gate insulating film 9 atthe bottom of the trench 16 is reduced.

Further, the n⁺-type region 17 is provided beneath (direction from thesource electrode pad 14 toward the drain electrode pad 15) portions ofthe first p⁺-type base region 4 between the trenches 16, therebyenabling the position where electric field concentrates to becontrolled. Furthermore, as compared to providing the n⁺-type region 17beneath the first p⁺-type base region 4 overall, a similar effect may beobtained by a lower nitrogen dose amount and since the dose amount islower, the number of ion implantation stages may be reduced.

As depicted in FIGS. 1 and 2, in the p-type base layer 6, an n⁺-typesource region (second semiconductor region of the first conductivitytype) 7 is provided at the first main surface side of the siliconcarbide semiconductor base. A p⁺⁺-type contact region (thirdsemiconductor region of the second conductivity type) 8 may beselectively provided. Further, the n⁺-type source region 7 and thep⁺⁺-type contact region 8 are in contact with each other. Further, thep⁺⁺-type contact region 8 is selectively provided along the direction inwhich the striped shape of the trench 16 extends (refer to FIG. 18). Forexample, as depicted in FIGS. 1 and 2, the p⁺⁺-type contact region 8 maybe provided in only portions where the first p⁺-type base region 4 isnot provided.

An interlayer insulating film 11 is provided at the first main surfaceside of the silicon carbide semiconductor base overall, so as to coverthe gate electrode 10 embedded in the trench 16. A source electrode(first electrode) 12, via a contact hole opened in the interlayerinsulating film 11, is in contact with the n⁺-type source region 7 andthe p⁺⁺-type contact region 8 (when the p⁺⁺-type contact region 8 is notprovided, the p-type base layer 6). The source electrode 12 iselectrically insulated from the gate electrode 10 by the interlayerinsulating film 11. On the source electrode 12, the source electrode pad14 is provided.

In FIGS. 1 and 2, while only two cells (a structure formed by the trench16, the gate insulating film 9, the gate electrode 10, the interlayerinsulating film 11 and the source electrode 12) are depicted, more cellsof a MOS gate (insulated gate formed by a metal, an oxide film, and asemiconductor) structure may be further disposed in parallel.

A method of manufacturing a silicon carbide semiconductor deviceaccording to an embodiment will be described. FIGS. 4, 5A, 5B, 6A, 6B,7A, 7B, 8, and 9 are cross-sectional views of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture. Here, FIG. 5A, FIG. 6A, and FIG. 7A are cross-sectionalviews of the silicon carbide semiconductor device according to thepresent embodiment during manufacture and correspond to FIG. 1. FIGS.5B, 6B, and 7B are cross-sectional views of the silicon carbidesemiconductor device according to the present embodiment duringmanufacture and correspond to FIG. 2.

First, the n⁺-type silicon carbide substrate 1 containing an n-typesilicon carbide is prepared. Then, on the first main surface side of then⁺-type silicon carbide substrate 1, the n⁻-type silicon carbideepitaxial layer 2 containing silicon carbide is formed by epitaxialgrowth to have a thickness of, for example, about 30 μm while an n-typeimpurity, for example, nitrogen atoms (N), is doped. The state up tohere is depicted in FIG. 4.

Next, at the surface of the n⁻-type silicon carbide epitaxial layer 2,an n-type impurity, for example, nitrogen atoms, may be ion implanted byan ion implantation method. As a result, in a surface layer of then⁻-type silicon carbide epitaxial layer 2, a first n-type region 5 ahaving, for example, a depth of 0.5 μm or less is formed. A dose amountduring ion implantation for forming the first n-type region 5 a, forexample, may be set so that the impurity concentration becomes about1×10¹⁷/cm³.

Next, on the surface of the first n-type region 5 a (2), a non-depictedmask having predetermined openings is formed by a photolithographytechnique using, for example, an oxide film. Then, using the oxide filmas a mask, an n-type impurity, for example, nitrogen atoms, is ionimplanted by an ion implantation method. As a result, in a surface layerof the first n-type region 5 a (2), the n⁺-type region 17 is formed.Further, the n⁺-type region 17, similarly to the first n-type region 5 a(2), is formed by ion implantation of nitrogen atoms and therefore, maybe formed by self-alignment with the first n-type region 5 a (2).

Next, the mask used during the ion implantation for forming the n⁺-typeregion 17 is removed. Then, on the surface of the first n-type region 5a (2) and the surface of the n⁺-type region 17, a non-depicted maskhaving predetermined openings is formed by a photolithography techniqueusing, for example, an oxide film. Then, using the oxide film as a mask,a p-type impurity, for example, aluminum atoms (Al), is ion implanted byan ion implantation method. As a result, at surface regions of then⁺-type region 17 and the first n-type region 5 a (2) in the surfacelayer of the n⁻-type silicon carbide epitaxial layer 2, the firstp⁺-type base region 4 having, for example, a depth of about 0.5 μm isformed. A dose amount during ion implantation for forming the firstp⁺-type base region 4, for example, may be set so that the impurityconcentration becomes about 5×10¹⁸/cm³. The state up to here is depictedin FIGS. 5A and 5B. Here, FIG. 5A is a portion in which the firstp⁺-type base region 4 is provided along the direction orthogonal to thedirection in which the striped shape of the trench 16 extends; FIG. 5Bis a portion in which the first p⁺-type base region 4 is not providedalong the direction orthogonal to the trench 16.

Next, the mask used during the ion implantation for forming the firstp⁺-type base region 4 is removed. Then, on the surface of the firstn-type region 5 a (2) and the surface of the first p⁺-type base region4, a second n-type region 5 b (2) containing silicon carbide is formedby epitaxial growth to have a thickness of, for example, about 30 μmwhile an n-type impurity, for example, nitrogen atoms, is doped. Thefirst n-type region 5 a (2) and the second n-type region 5 b (2)collectively form the n-type high-concentration region 5 (2). The stateup to here is depicted in FIGS. 6A and 6B.

Next, on the surface (i.e., surface of the second n-type region 5 b (2))of the n⁻-type silicon carbide epitaxial layer 2, the p-type base layer6 is formed by epitaxial growth to have a thickness of, for example,about 1.3 μm while a p-type impurity, for example, aluminum atoms, isdoped. Conditions of epitaxial growth for forming the p-type base layer6 may be set so that, for example, the impurity concentration becomesabout 4×10¹⁷/cm³ lower than an impurity concentration of a secondp⁺-type base region (second base region) 3. By the processes up to here,the silicon carbide semiconductor base in which the n⁻-type siliconcarbide epitaxial layer 2, the n-type high-concentration region 5 (2),and the p-type base layer 6 are stacked on the n⁺-type silicon carbidesubstrate 1 is formed.

Next, on the surface of the p-type base layer 6, a non-depicted maskhaving predetermined openings is formed by a photolithography techniqueusing, for example, an oxide film. Then, using the oxide film as a mask,an n-type impurity, for example, phosphorus (P) is ion implanted by anion implantation method. As result, at a portion of the surface of thep-type base layer 6, the n⁺-type source region 7 is formed. A doseamount during ion implantation for forming the n⁺-type source region 7may be set so that, for example, the impurity concentration becomeshigher than that of the second p⁺-type base region 3.

Next, the mask used during ion implantation for forming the n⁺-typesource region 7 is removed. Then, on the surface of the p-type baselayer 6, a non-depicted mask having predetermined openings is formed bya photolithography technique using, for example, an oxide film. Usingthe oxide film as a mask, a p-type impurity, for example, aluminum, ision implanted at the surface of the p-type base layer 6. As a result,the p⁺⁺-type contact region 8 may be formed at a portion of surfaceregions of the p-type base layer 6. A dose amount during ionimplantation for forming the p⁺⁺-type contact region 8, for example, maybe set so that the impurity concentration becomes higher than that ofthe p-type base layer 6. Subsequently, the mask used during ionimplantation for forming the p⁺⁺-type contact region 8 is removed. Asequence in which the ion implantation for forming the n⁺-type sourceregion 7 and the ion implantation for forming the p⁺⁺-type contactregion 8 are performed may be interchanged. The state up to here isdepicted in FIGS. 7A and 7B.

Next, a heat treatment (annealing) is performed, thereby activating, forexample, the first p⁺-type base region 4, the n⁺-type source region 7,the p⁺⁺-type contact region 8, and the n⁺-type region 17. A temperatureof the heat treatment may be, for example, about 1700 degrees C. Aperiod of the heat treatment may be, for example, about 2 minutes. Ionimplanted regions may be collectively activated by one session of theheat treatment as described above or may be activated by performing theheat treatment each time ion implantation is performed.

Next, on the surface (i.e., surfaces of the n⁺-type source region 7 andthe p⁺⁺-type contact region 8) of the p-type base layer 6, anon-depicted mask having predetermined openings is formed by aphotolithography technique using, for example, an oxide film. Then, thetrench 16 that penetrates the n⁺-type source region 7 and the p-typebase layer 6, and reaches the n-type high-concentration region 5 (2) isformed by dry etching using the oxide film as a mask. The bottom of thetrench 16 reaches the first p⁺-type base region 4. Subsequently, themask used for forming the trench 16 is removed. The state up to here isdepicted in FIG. 8. In FIGS. 8 and 9, formation of the trench 16 iscommon to the configurations in FIGS. 1 and 2 and therefore, only across-sectional view during manufacturing corresponding to FIG. 1 isdepicted.

Next, the gate insulating film 9 is formed on the surfaces of then⁺-type source region 7 and the p⁺⁺-type contact region 8 and along thebottom and side walls of the trench 16. The gate insulating film 9 maybe formed by thermal oxidation by a heat treatment at a temperature of1000 degrees C. in an oxygen atmosphere. Further, the gate insulatingfilm 9 may be formed by a deposition method by a chemical reaction suchas that for a high temperature oxide (HTO).

Next, on the gate insulating film 9, a polycrystalline silicon layerdoped with, for example, phosphorus atoms is formed. The polycrystallinesilicon layer is formed so as to be embedded in the trench 16. Thepolycrystalline silicon layer is patterned and left in the trench 16,thereby forming the gate electrode 10. A portion of the gate electrode10 may protrude from a top (side where the source electrode pad 14 isprovided) of the trench 16, toward the source electrode pad 14.

Next, a phosphate glass is deposited so as to cover the gate insulatingfilm 9 and the gate electrode 10 and have a thickness of, for example,about 1 μm, thereby forming the interlayer insulating film 11. Theinterlayer insulating film 11 and the gate insulating film 9 arepatterned and selectively removed, whereby contact holes are formed,exposing the n⁺-type source region 7 and the p⁺⁺-type contact region 8(when the p⁺⁺-type contact region 8 is not formed, the p-type base layer6). Thereafter, a heat treatment (reflow) is performed, therebyplanarizing the interlayer insulating film 11. The state up to here isdepicted in FIG. 9.

Here, in the contact holes and on the interlayer insulating film 11, aconductive film that forms the source electrode 12 is formed. Theconductive film is selectively removed, for example, leaving the sourceelectrode 12 only in the contact hole.

Subsequently, at the second main surface of the n⁺-type silicon carbidesubstrate 1, the rear electrode 13 is formed, for example, using anickel (Ni) film. Thereafter, a heat treatment is performed at atemperature of, for example, about 970 degrees C., whereby the n⁺-typesilicon carbide substrate 1 and the rear electrode 13 form an ohmicjunction.

Next, for example, by a sputtering method, for example, an aluminum filmis provided so as to cover the source electrode 12 and the interlayerinsulating film 11, and have a thickness of, for example, about 5 μm.Thereafter, the aluminum film is selectively removed and left so as tocover an active portion of the element overall, whereby the sourceelectrode pad 14 is formed. Here, the active portion is a portion wherethe element structure is formed and through which current passes in anON state.

Next, at the surface of the rear electrode 13, for example, titanium(Ti), nickel, and gold (Au) are sequentially stacked, whereby the drainelectrode pad 15 is formed. Thus, as described, the semiconductor devicedepicted in FIGS. 1 and 2 is completed.

As described above, according to the first embodiment, the n⁺-typeregion having a peak impurity concentration higher than the n-typehigh-concentration region is provided at a position between the trenchesand deeper than the first p⁺-type base region. As a result, thebreakdown voltage of the first p⁺-type base region of the bottom of thetrench may be set higher than the breakdown voltage of the first p⁺-typebase region between the trenches, thereby selectively causing avalanchebreakdown in the n⁺-type region and enabling the occurrence of avalanchebreakdown at the bottom of the trench to be suppressed. In this manner,avalanche current is not generated at the bottom of the trench andtherefore, insulation reliability of the gate insulating film ismaintained.

FIG. 10 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a second embodiment, along cuttingline A-A′ in FIG. 12. FIG. 11 is a cross-sectional view of the structureof the silicon carbide semiconductor device according to the secondembodiment, along cutting line B-B′ in FIG. 12. FIG. 12 is a plansectional view of the structure of the silicon carbide semiconductordevice according to the second embodiment. The silicon carbidesemiconductor device according to the second embodiment differs from thesilicon carbide semiconductor device according to the first embodimentin that the second p⁺-type base region 3 is included, the second p⁺-typebase region 3 being parallel to the direction in which the striped shapeof the trench 16 extends and in contact with the bottom of the trench16.

The second p⁺-type base region 3, similarly to the first p⁺-type baseregion 4, enables a pn junction between the second p⁺-type base region 3and the n-type high-concentration region 5 to be formed at a positionnear the bottom of the trench 16 in the depth direction and enables highelectric field to be prevented from being applied to the gate insulatingfilm 9 at the bottom of the trench 16. Further, the second p⁺-type baseregion 3 is provided to have a width greater than the trench width andthus, enables mitigation of the electric field at a corner portion whereelectric field of the bottom of the trench 16 concentrates, whereby thebreakdown voltage may be further increased.

Further, in the second embodiment, the first p⁺-type base region 4 andthe second p⁺-type base region 3 are not provided in the depth directionof the p⁺⁺-type contact region 8 and therefore, reduction of the cellpitch is facilitated. Further, by the second p⁺-type base region 3, thebottom of the trench 16 is entirely covered by a p-type region, therebyenabling easier and more stable control of the breakdown voltage ascompared to the first embodiment.

Further, the first p⁺-type base region 4 and the second p⁺-type baseregion 3 may have the same impurity concentrations. Furthermore, theinterval w2 between the first p⁺-type base regions 4 may be a width(w2≈w4) about equal to an interval w4 between the second p⁺-type baseregions 3. Further, the interval w4 between the second p⁺-type baseregions 3 is a distance between a center of the second p⁺-type baseregion 3 and a center of an adjacent second p⁺-type base region 3 thatis adjacent to the second p⁺-type base region 3. Furthermore, a width w5of the first p⁺-type base region 4 or a width w6 of the second p⁺-typebase region 3 may be greater than a width w7 of the n⁺-type region 17(w5>w7, or w6>w7). The n⁺-type region 17 may be formed by ionimplantation of nitrogen without use of an oxide film mask andtherefore, the first p⁺-type base region 4 may be formed smaller,thereby enabling cell pitch to be reduced.

The silicon carbide semiconductor device according to the secondembodiment may be manufactured by concurrently forming the secondp⁺-type base region 3 with the first p⁺-type base region 4 in the methodof manufacturing the silicon carbide semiconductor device according tothe first embodiment.

As described above, according to the second embodiment, effects similarto those of the first embodiment may be obtained. Further, in the secondembodiment, the second p⁺-type base region that is parallel to thedirection in which the striped shape of the trench extends is includedat the bottom of the trench and therefore, all of the bottom of thetrench is covered by a p-type region, thereby enabling more stable andeasier control of the breakdown voltage as compared to the firstembodiment.

FIG. 13 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a third embodiment. In the thirdembodiment, a plan sectional view is similar to that of the firstembodiment and therefore, is not depicted (refer to FIG. 3A). Further, across-sectional view of a portion where the first p⁺-type base region 4is not provided is similar to that of the first embodiment andtherefore, is not depicted (refer to FIG. 2).

The silicon carbide semiconductor device according to the thirdembodiment differs from the silicon carbide semiconductor deviceaccording to the first embodiment in that the n⁺-type region 17 isprovided at a position facing the p⁺⁺-type contact region 8 in the depthdirection. In other words, the n⁺-type region 17 is provided onlybeneath the p⁺⁺-type contact region 8. As such, the ON resistance may bereduce more by the n⁺-type region 17 that in the first embodiment.

The silicon carbide semiconductor device according to the thirdembodiment may be manufactured by forming the n⁺-type region 17 onlybeneath a formation region of the p⁺⁺-type contact region 8 when then⁺-type region 17 is formed in the method of manufacturing the siliconcarbide semiconductor device according to the first embodiment.

As described above, according to the third embodiment, effects similarto those of the first embodiment may be obtained. Further, in the thirdembodiment, the n⁺-type region 17 is provided only beneath the p⁺⁺-typecontact region 8, thereby enabling the ON resistance to be reduced morethan in the first embodiment.

FIG. 14 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a fourth embodiment, along cuttingline B-B′ in FIG. 15. FIG. 15 is a plan sectional view of the structureof the silicon carbide semiconductor device according to the fourthembodiment. Further, a cross-sectional view along cutting line A-A′ inFIG. 15, where the first p⁺-type base region 4 is provided is similar tothat of the first embodiment and therefore, is not depicted (refer toFIG. 1).

The silicon carbide semiconductor device according to the fourthembodiment differs from the silicon carbide semiconductor deviceaccording to the first embodiment in that the n⁺-type region 17 isprovided in a striped shape parallel to the direction in which thestriped shape of the trench 16 extends. With such a shape, alignmentbecomes unnecessary when the n⁺-type region 17 is formed only beneaththe first p⁺-type base region 4 and process difficulty is reduced.

The silicon carbide semiconductor device according to the fourthembodiment may be manufactured by forming, in the method ofmanufacturing the silicon carbide semiconductor device according to thefirst embodiment, the n⁺-type region 17 in a striped shape parallel tothe direction in which the striped shape of the trench 16 extends.

As described above, according to the fourth embodiment, effects similarto those of the first embodiment are obtained. Further, in the fourthembodiment, alignment when the first p⁺-type base region and the n⁺-typeregion are formed becomes unnecessary and process difficulty is reduced.

FIG. 16 is a cross-sectional view of a structure of the silicon carbidesemiconductor device according to a fifth embodiment. A cross-sectionalview of a portion in which the first p⁺-type base region 4 is notprovided is similar to that of the first embodiment and therefore, notdepicted (refer to FIG. 2). A plan sectional view of the structure ofthe silicon carbide semiconductor device according to the fifthembodiment is similar to that of the first embodiment and therefore, isnot depicted (refer to FIG. 3A).

The silicon carbide semiconductor device according to the fifthembodiment differs from the silicon carbide semiconductor deviceaccording to the first embodiment in that the n⁺-type region 17 isprovided in the n-type high-concentration region 5 (2). Even with suchas shape, the n⁺-type region 17 enables the breakdown voltage of thefirst p⁺-type base region 4 at the bottom of the trench 16 to be madehigher than the breakdown voltage of the first p⁺ type base region 4between the trenches 16 and prevents avalanche beneath the trench,enabling reliability to be increased.

Further, a distance h1 between the n⁺-type region 17 and the firstp⁺-type base region 4 may be at least smaller than a thickness h2 of thefirst p⁺-type base region 4 (h1<h2) because when equal to or greaterthan the thickness h2 of the first p⁺-type base region 4, separation isexcessive, whereby electric field cannot be concentrated at the n⁺-typeregion 17.

The silicon carbide semiconductor device according to the fifthembodiment may be manufactured by forming the n⁺-type region 17 in then-type high-concentration region 5 (2) when forming the n⁺-type region17 in the method of manufacturing the silicon carbide semiconductordevice according to the first embodiment.

As described above, according to the fifth embodiment, effects similarto those of the first embodiment may be obtained.

In the present invention, while a case in which the first main surfaceof the silicon carbide substrate containing silicon carbide is a (0001)plane and on the (0001) plane, a MOS gate structure is configured hasbeen described as an example, without limitation hereto, variousmodifications are possible such as in the type (for example, galliumnitride (GaN) or the like) of the wide bandgap semiconductor, surfaceorientation of the substrate main surface, etc.

Further, in the embodiments of the present invention, while the firstconductivity type is an n-type and the second conductivity type is ap-type, the present invention is similarly implemented when the firstconductivity type is a p-type and the second conductivity type is ann-type.

According to the invention described above, the n⁺-type region (firstsemiconductor region of the first conductivity type) having a peakimpurity concentration higher than the n-type high-concentration region(the first semiconductor layer of the first conductivity type) isprovided between the trenches, at a position deeper than the firstp⁺-type base region (first base region of the second conductivity type).As a result, the breakdown voltage of the first p⁺-type base region atthe bottom of the trench may be made higher than the breakdown voltageof the first p⁺-type base region between the trenches, selectivelycausing avalanche breakdown in the n⁺-type region and enabling theoccurrence of the avalanche breakdown at the bottom of the trench to besuppressed. In this manner, avalanche current is not generated at thebottom of the trench and therefore, insulation reliability of the gateinsulating film is maintained.

According to the semiconductor device according to the present inventionand the method of manufacturing the semiconductor device, an effect isachieved in that the occurrence of avalanche breakdown at the bottom ofthe trench may be suppressed.

In this manner, the semiconductor device according to the presentinvention and the method of manufacturing the semiconductor device isuseful for high-voltage semiconductor devices used in power convertingequipment and power source devices such as in various industrialmachines.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor substrate of a first conductivity type, having a frontsurface and a back surface opposite to each other; a first semiconductorlayer of the first conductivity type, provided on the front surface ofthe semiconductor substrate and having a first side facing thesemiconductor substrate and a second side opposite to the first side, animpurity concentration of the first semiconductor layer being lower thanthat of the semiconductor substrate, the first semiconductor layerincluding a high-concentration region of the first conductivity type; afirst semiconductor region of the first conductivity type, selectivelyprovided in the first semiconductor layer, a peak impurity concentrationof the first semiconductor region being higher than that of the firstsemiconductor layer; a first base region of a second conductivity type,selectively provided in the first semiconductor layer; a secondsemiconductor layer of the second conductivity type, provided on asurface of the first semiconductor layer at the second side thereof; asecond semiconductor region of the first conductivity type, selectivelyprovided in the second semiconductor layer at a surface thereof; atrench that penetrates the second semiconductor layer and the secondsemiconductor region, and reaches the first semiconductor layer; a gateelectrode provided in the trench, via a gate insulating film; aninterlayer insulating film provided on the gate electrode; a firstelectrode in contact with the second semiconductor layer and the secondsemiconductor region; and a second electrode provided on the backsurface of the semiconductor substrate, wherein the trench includes aplurality of trench portions, each of which extends in a firstdirection, the first semiconductor region is provided between adjacenttwo of the plurality of trench portions, and is closer to the secondelectrode than is the first base region, and the first base region is indirect contact with a bottom of the trench, and extends continuouslyalong a second direction that intersects the first direction, at a depththat is the same as the bottom of the trench, or is closer to the secondelectrode than is the bottom of the trench.
 2. The semiconductor deviceaccording to claim 1, wherein the first base region includes a pluralityof first base region portions, and an interval between any adjacent twoof the first base region portions is shorter than an interval betweenany adjacent two of the trench portions.
 3. The semiconductor deviceaccording to claim 1, wherein the first base region includes a pluralityof first base region portions, and an interval between any adjacent twoof the first base region portions is in a range from 0.8 μm to 5 μm. 4.The semiconductor device according to claim 1, wherein an impurityconcentration of the first semiconductor region is lower than animpurity concentration of the first base region.
 5. The semiconductordevice according to claim 1, wherein an impurity concentration of thefirst semiconductor region is in a range from 1×10¹⁷/cm³ to 1×10²⁰/cm³.6. The semiconductor device according to claim 1, wherein an anglebetween the first and second directions is one of 30 degrees, 60degrees, 90 degrees, 120 degrees or 150 degrees.
 7. The semiconductordevice according to claim 6, wherein the angle has a margin of ±4degrees.
 8. The semiconductor device according to claim 1, wherein thefirst base region is separated from the second semiconductor layer bythe first semiconductor layer.
 9. The semiconductor device according toclaim 1, wherein the first semiconductor region is separated from thesecond semiconductor layer by the first semiconductor layer and thefirst base region.
 10. A method of manufacturing a semiconductor device,the method comprising: providing a semiconductor substrate of a firstconductivity type, the semiconductor substrate having a front surfaceand a back surface opposite to each other; forming, on the front surfaceof the semiconductor substrate, a first semiconductor layer of the firstconductivity type, the first semiconductor layer having an impurityconcentration lower than that of the semiconductor substrate and havinga first side facing the semiconductor substrate and a second sideopposite to the first side, the first semiconductor layer including ahigh-concentration region of the first conductivity type; selectivelyforming, in the first semiconductor layer, a first semiconductor regionof the first conductivity type, the first semiconductor region having apeak impurity concentration higher than that of the first semiconductorlayer; selectively forming, in the first semiconductor layer, a firstbase region of a second conductivity type; forming, on a surface of thefirst semiconductor layer at the second side thereof, a secondsemiconductor layer of the second conductivity type; selectivelyforming, in the second semiconductor layer at a surface thereof, asecond semiconductor region of the first conductivity type; forming atrench that penetrates the second semiconductor layer and the secondsemiconductor region, and reaches the first semiconductor layer;forming, in the trench, a gate electrode via a gate insulating film;forming an interlayer insulating film on the gate electrode; forming afirst electrode in contact with the second semiconductor layer and thesecond semiconductor region; and forming a second electrode on the backsurface of the semiconductor substrate, wherein forming a trenchincludes forming a plurality of trench portions, each of which extendsin a first direction, the first base region being in direct contact witha bottom of the trench, forming the first semiconductor region includesforming a plurality of first semiconductor region portions, each betweenadjacent two of the trench portions, the first semiconductor regionbeing closer to the second electrode than is the first base region, andforming the first base region includes forming the first base region toextend continuously along a second direction intersecting the firstdirection, at a depth that is the same as the bottom of the trench, oris closer to the second electrode than is the bottom of the trench. 11.The method according to claim 10, wherein the first base region isformed to be separated from the second semiconductor layer by the firstsemiconductor layer.
 12. The method according to claim 10, wherein thefirst semiconductor region is formed to be separated from the secondsemiconductor layer by the first semiconductor layer and the first baseregion.
 13. The semiconductor device according to claim 1, wherein thehigh-concentration region extends from the second semiconductor layer toa depth deeper than that of the first semiconductor region, an impurityconcentration of the high-concentration region being lower than that ofthe semiconductor substrate, being lower than that of the firstsemiconductor region, and being higher than that of the firstsemiconductor layer.